41
votes
Accepted
What's the difference between Inactive and HoldForm?
Updated
Both Hold and Inactive block evaluation; the key difference is that Inactive is ...
- 8,372
25
votes
Why is Unevaluated[#]& different from Unevaluated?
I thought that for any function f we could use "f" interchangeably with "f[#]&".
That ...
- 225k
23
votes
Accepted
What are the cool kids talking about when they use ##&[]?
Try this:
Map[If[#==1,Unevaluated@Sequence[],#]&,{1,2,3}]
Note the output. The 1 is gone. That's because ...
- 25.1k
23
votes
Accepted
What's the difference between these 2 associations?
Preamble
The real problem here seems somewhat deeper than what the (mostly correct) observations in comments indicate. In Mathematica, a number of objects, which are so-called raw objects (including ...
- 112k
21
votes
Accepted
Can't inject EncryptedObject
I think the problem might be related to a bug in FullForm when applied to a ByteArray object:
...
- 67k
19
votes
What's the difference between Inactive and HoldForm?
One difference is that NDSolve directly supports Inactive. It can be used to specify operators such as divergence ($\nabla\cdot$...
- 225k
18
votes
Accepted
Why is Unevaluated[#]& different from Unevaluated?
The two are not equivalent. Even if f lacks special attributes.
Assuming no special attributes are present, a function's arguments are always evaluated before the ...
- 1,267
18
votes
Accepted
Why does Unevaluated applied (@@) to a held expression not work?
The action of the symbol Unevaluated is due to a special rule within the evaluator that occurs at a very specific time during the evaluation process. ...
- 67k
16
votes
Accepted
Pure functions are not functions?
A response to this comment:
I understand that there are workarounds. I am wondering whether I am misinterpreting the manual.
Yes. The use of the word "function" in the manual may be confusing ...
- 225k
16
votes
Accepted
How to uncompress strings safely (without any evaluation)?
This is definitely a bug. Here's a slightly simpler workaround than those suggested in the comments:
Uncompress[str, HoldComplete] // InputForm
Generally ...
- 13.4k
16
votes
Accepted
Easy printing of executable code
You could define a function similar to Echo with the difference that it won't evaluate its argument before printing.
...
- 225k
16
votes
Why is Unevaluated[#]& different from Unevaluated?
The head Unevaluated is a symbol with the HoldAllComplete attribute. The head ...
- 13.5k
16
votes
Accepted
Evaluated vs. unevaluated Association
Summary
The confusion we observe here is largely due to ambiguous use of the symbol Association as an expression head. On the one hand, ...
- 67k
14
votes
Accepted
What is the potential usage of ReplaceAllUnheld in Developer Utilities Package?
Thank you for bringing this function to my attention! It lets us use ReplaceAll without disturbing or even traversing held expressions. This is huge! I have ...
- 264k
14
votes
Accepted
HoldPattern vs Unevaluated
HoldPattern is a persistent head that prevents evaluation of its argument(s) yet is transparent to pattern matching. Unevaluated...
- 264k
13
votes
Accepted
Pattern does not match with Orderless head
I am betting that this is almost certainly an optimization short-cut. If Orderless had to try every ordering it would be extremely slow when there are a moderate ...
- 264k
13
votes
Accepted
Defer, Hold, Unevaluated, Inactivate -- which one when?
The SO duplicate Preventing evaluation of Mathematica expressions
does a nice job of explaining the differences among and use-cases for Defer, ...
- 21.6k
12
votes
What's the difference between Inactive and HoldForm?
Although mfvonh's answer is a nice summary of Inactive formal properties, I think it misses several important points, which are both shown in the "Scope" and "...
- 2,661
12
votes
Accepted
How to properly inject iterators into table?
General
I think that one can achieve the goal much easier if we reformulate the request. A variable is IMO not a proper object to store an iterator in the form of expression. What you really need is ...
- 112k
11
votes
Accepted
How to see a code preview (in Experimental`Explore[] or related GUI)
Quick fix is to use e.g. Inactivate and friends:
...
Kuba♦
- 133k
11
votes
Accepted
Why one needs two Unevaluated to show 1+1 correctly in TreeForm?
This issue is known and it is a defect of TreeForm like Leonid already said. Let me give an illustrative example that shows the same behavior. Let us assume you ...
- 111k
11
votes
Accepted
Matching HoldPattern expressions
Clarification of HoldPattern usage:
HoldPattern[expr] is equivalent to expr for pattern ...
Kuba♦
- 133k
11
votes
Accepted
How to get a unevaluted expression from a compressed string?
You need to wrap the expression Unevaluated before passing it to Compress, which means defining your own function instead of ...
- 17.9k
11
votes
Accepted
Confusion about HoldFirst attribute of Set
The problem is not related to Set in general but to how Set works with Part on a left hand ...
Kuba♦
- 133k
11
votes
Expanding a sum without evaluating it
may be
Sum[With[{i=i},HoldForm[i^2]],{i,1,5}]
But this uses a wrapper. I do not think there is a way without using Hold and ...
- 110k
11
votes
Accepted
Perform localized, evaluation-leak free replacements
Here's a way making use of Block to cause f to be inert:
...
- 45.5k
11
votes
Accepted
Getting terms and only evaluate specific parts of a series
times[{i_}] := i
times[{i__}] := Inactive[Times][i]
Sum[times@Table[2 i - 1, {i, n}]/times@Table[2 i, {i, n}], {n, 5}]
If you prefer ...
- 52.7k
10
votes
What's the difference between Inactive and HoldForm?
I think the basic answer to your question is:
1) If you want absolute control over revaluation, nothing beats good old HoldComplete. It will be there to the end of time.
2) Inactive meant for much ...
- 13.4k
10
votes
Accepted
Held keys in associations
Wrap the entire key in Hold, if you really want to keep things that might evaluate in the keys of an association (HoldPattern is ...
- 10.5k
10
votes
Accepted
HoldForm does not Hold Form for fractions sometimes
Use HoldForm applied to each fraction to keep the fractions from combining.
HoldForm[1/2] HoldForm[3/4]
to produce $$ \frac{1}...
- 58.4k
Only top scored, non community-wiki answers of a minimum length are eligible